Aircraft utilizing multiple propeller/prop-fan propulsion systems are faced with an acoustic noise and vibration problem induced by each of the individual propellers acting independently and creating airflow disturbances which interact randomly with one another and impinge on the aircraft fuselage with varying intensities over time. The known synchrophasing art involves designating one propeller as the master and the remaining propellers as slaves and attempting to make each slave maintain a precise angular relationship with respect to the position of the master in order to provide optimum noise and vibration cancellation. The best combination of propeller phase relations normally is established by flight testing.
Known synchrophasers adjust a slave propeller's blade angle pitch and hence the propeller's speed to maintain the proper phase relations. Engine power is maintained constant in all engines. If a slave starts to advance or get ahead of the master its pitch is increased in order to increase its power absorption. But since power is maintained constant its speed drops off instead. In this way adjustable pitch can be used to maintain phase by controlling speed. A problem with this approach is that the amount of speed change and hence the amount of blade movement actually needed to restore an advancing or retarding slave propeller is miniscule. Unfortunately, for small pitch change command signals the typical speed governor is nonresponsive. Small signal non-linearities such as hysteresis and dead zone are inherent in propeller pitch change systems. Thus, for example, an advancing or retarding slave blade will continue to advance or fall behind the master until the magnitude of the error signal is large enough to get out of the dead band of the governor. The basic problem is that the pitch control system is a high power servo not responsive to small amplitude command signals. And the consequence is that the phase error continues to grow until it gets the blade angle to move. Once the blade responds it continues past the desired point through its dead zone and just keeps going an equal amount in the other direction until the phase error once again grows large enough to exit the dead band, albeit on the other side. This is the well-known phenomenon called "limit cycling," i.e., the inability to hold the phase angle accurately because of the inability to overcome the servo's hysteresis with small command signals.
The prior art attempts to address this problem by adding "dither" to the pitch change actuator command signal. The frequency of the superimposed dither signal is high relative to the frequency response of the system and the result is that the dead band is narrowed to a considerable extent. For example, a synchrophaser which has the capability of correcting changes at the rate of 1/6 cycle per second might have a 21/2 hertz dither signal superimposed. The dither action imposes an artifical command signal excursion outside the dead band in response to which the system is continually attempting, unsuccessfully, to keep up. However, the system's lack of success in fully following the amplitude of the fast dither commands has a side benefit in keeping the blade actuator "moving" about an average command signal level. This has the effect of narrowing the width of the dead band. A synchrophaser in which the phase error might grow to as much as 12.degree. without any dither is improved to only 3.degree. with dither. A shortcoming of the dither approach, however, is that in asking the blade angle to be moving almost continuously, back and forth, a high degree of wear on seals and other loaded surfaces in the actuator results.